Yeast cells contain several ABC (ATP-binding cassette) transmembrane proteins that mediate broad- spectrum drug resistance. The Pdr5p multidrug transporter is required for resistance to a large range of chemically and mechanistically distinct substrates. Previous work by our group used several transport assays to establish that substrate size is critical for this interaction. We also demonstrated that Pdr5p has three substrate-binding sites and that the number and organization of electronegative groups is partly responsible for site specificity. Pdr5p-substrate specificity is different from its mammalian counterparts such as P-glycoprotein. In consultation with Dr. Di Xia, we used bioinformatics to define the core residues of the transmembrane helical domains (TMHs). The major goal of this proposal is to use site-directed mutagenesis to identify amino acids present in the TMHs that serve as hydrogen-bond donors at the Pdr5p substrate- binding sites. Very little is known about the molecular basis of substrate / multidrug-transporter interaction in eukaryotes. Such information is of considerable clinical relevance, however, as multidrug resistance is a major problem in the treatment of fungal and parasitic pathogens. An understanding of the molecular basis of substrate specificity may help in the design of better chemotherapeutic agents or inhibitors of transporter function. Successful implementation of this proposal will have impact beyond the training of graduate students. As described further, we have strong collaboration with Dr. Suresh V. Ambudkar and are now consulting with Dr. Di Xia (NIH/NCI/Laboratory of Cell Biology). We also train numerous undergraduates and summer interns. [unreadable] [unreadable] [unreadable]